Method and device for generating a synchronisation variable and the corresponding integrated circuit and digital disc drive

ABSTRACT

A synchronization variable intended for a second clock signal is generated from a first clock signal and a phase variation signal. A first approximation of the second clock signal is determined, and other approximations close to the first approximation are also determined. An error is calculated for each of the approximations, and the best approximation is taken as the second clock signal.

[0001] The invention concerns generating a variable for synchronizing one signal with another signal, in particular a clock signal.

[0002] The invention advantageously applies, although this is not limiting on the invention, to the field of digital discs, in particular those known as compact disc read-only memory (CD-ROM), and especially digital versatile discs (DVD) for storing compressed picture data.

[0003] Synchronization can be effected using an analog phase-locked loop, which is rather costly.

[0004] Synchronization can also be effected using a digital phase-locked loop. However, calculating a synchronization variable, such as the time interval between a transition of an approximate signal and a transition of the signal to be obtained, necessitates division operations that are slow and costly in terms of calculation resources.

[0005] The invention aims to propose a simple and precise solution to the problems of the prior art.

[0006] The invention proposes a method of generating a synchronization variable WCS intended for a second clock signal WP₂ from a first clock signal WP₁ and a phase variation signal ΔΨ, in which method:

[0007] a first approximation WS₀ of the synchronization variable is determined,

[0008] other approximations close to the first approximation are determined,

[0009] the error is calculated for each of the approximations, and

[0010] the best approximation is taken as the synchronization variable.

[0011] Prior to determining the first approximation WS₀ of the synchronization variable, an approximation P_(wp) of the period of the second clock signal WP₂ can be determined.

[0012] A second approximation WS_(p1) and a third approximation WS_(m1) of the synchronization variable are preferably determined, where WS_(p1)=WS₀+1 and WS_(m1)=WS₀−1.

[0013] In one embodiment of the invention the phase Ψ is calculated at a given time by adding phase variation signals ΔΨ of preceding times and of the given time modulo the period P_(wp1) of the clock signal WP₁.

[0014] In one embodiment of the invention the first approximation WS₀ is obtained from the following equation, in which A is a constant: WS₀=Ψ* P_(wp)/A.

[0015] In one embodiment of the invention error values are determined corresponding to the approximations effected:

err _(—)0=(B*Ψ)−(C*ΔΨ*WS₀)

err _(—) p1=(B*Ψ)−(C*ΔΨ*(WS ₀+1))

err _(—) m1=(B*Ψ)−(C*ΔΨ*(WS ₀−1)), where B and C are constants.

[0016] In one embodiment of the invention the synchronization variable WCS is equal to the approximation generating the smallest error.

[0017] The synchronization variable WCS is used to synchronize the clock signal WP₂ and the clock signal WP₁.

[0018] In one embodiment of the invention A is equal to 2^(n) where n is a predetermined constant and can be equal to 15.

[0019] The invention also proposes a device for generating a synchronization variable WCS intended for a second clock signal WP₂ from a first clock signal WP₁ and a phase variation signal ΔΨ, including:

[0020] means for determining a first approximation WS₀ of the synchronization variable,

[0021] means for determining other approximations close to the first approximation,

[0022] means for calculating the error for each of the approximations, and

[0023] means for taking the best approximation as the synchronization variable.

[0024] The invention also proposes an integrated circuit including the above device.

[0025] The invention also proposes a digital disc reader including an optical head including means for emitting an incident light beam and a plurality of photodetectors for detecting the reflection of the light beam at the disc, summing means for summing signals from the photodetectors two by two and delivering two sampled incident signals, and calculator means for calculating the phase difference of the two sampled incident signals. The calculator means include the above device.

[0026] The invention calculates a variable, from which a signal can be generated whose phase is determined with known accuracy, from the relative phase between two individual signals from photodetectors, which corresponds to the positioning error of an incident optical beam, for example a laser spot, illuminating the track on a digital disc. The optical system can then be modified to return the incident optical beam to the track and lock it there.

[0027] A digital disc has a single spiral track whose relief is representative of binary data stored on the track on the disc. The track on the disc is illuminated by an incident optical beam, for example a laser spot, and a plurality of photodetectors, for example four photodetectors, detect the reflection of the light beam on the disc.

[0028] The optical sensor formed by the photodetectors then supplies, on the one hand, four individual signals delivered by the respective four photodetectors, and which are also used to lock the optical beam onto the track on the disc, and, on the other hand, a global signal, or wanted signal, equal to the sum of the four individual signals, from which wanted signal binary data read from the track is extracted.

[0029] The code used for coding of the binary data on the disc is-standardized and well known to the skilled person (for example the RLL (2, 10) code). To be more precise, the lengths of the pits and bosses on the spiral track on the disc determine the number of logic 0 values bracketing two logic 1 values. The lengths of the pits and bosses are all multiples of a basic length, commonly referred to by the skilled person as the basic length 1T. For example, the basic length 1T is 0.64 micron for a DVD and 1.6 microns for a CD-ROM.

[0030] When the digital disc is rotating, the wanted signal containing the binary data, also referred to hereinafter as the “incident signal”, includes a succession of transitions whose spacings are representative of the lengths of said pulses. The higher the rotation speed of the disc, the smaller the spacings between the transitions.

[0031] The general principle of extracting the binary data conveyed by the incident signal then consists of detecting the transitions of the incident signal, calculating the distances between the successive transitions, and determining the data from the calculated distances.

[0032] The relative phase of two sampled incident signals can be calculated by an electronic device including:

[0033] a detector stage adapted to detect the transitions of each incident signal relative to a predetermined threshold as well as so-called “minimum” samples of that signal whose levels are below a bottom threshold and so-called “maximum” samples whose levels are above a top threshold,

[0034] a first pair of blocks associated with a first incident signal and a second pair of blocks associated with a second incident signal, each block including, on the one hand, memory elements adapted to store the levels of a predetermined set of samples of the corresponding incident signal, that set including at least one minimum sample, one maximum sample, and two intermediate samples representative of a transition of said incident signal temporally located between the minimum and maximum samples, and, on the other hand, first, second and third time counters clocked by the sampling clock signal and respectively associated with the minimum and maximum samples and one of the intermediate samples,

[0035] control means adapted, in the presence of two minimum samples or two maximum samples respectively relating to two incident signals, storing the first set of samples relating to the first signal in one of the blocks of the first pair and the first set of samples relating to the second signal in the like block of the second pair, and then storing the subsequent successive sets of samples of each incident signal alternately and successively in the two blocks of each pair, starting with the other block of each pair, initializing the time counter associated with the stored sample, and delivering a block validation signal when the corresponding set of samples has been stored completely in the memory elements of the block, and

[0036] post-processing means adapted to determine the relative phase of the two incident signals in the presence of two block validation signals relating to two like blocks of two pairs, at least from values of the third time counters of said two blocks and the levels of the intermediate samples stored in said two blocks.

[0037] This kind of electronic device for calculating a relative phase can be used upstream of the device for generating a synchronization variable to supply it with a phase variation signal ΔΨ.

[0038] The present invention will be better understood after reading the following detailed description of one embodiment of the invention, which description is given by way of non-limiting example only and refers to the accompanying drawings, in which:

[0039]FIG. 1 is a diagrammatic view of one embodiment of a device in accordance with the invention;

[0040]FIG. 2 is a flowchart of method steps;

[0041]FIG. 3 is a diagrammatic view of signals; and

[0042]FIG. 4 is a partial diagrammatic view of the internal architecture of a digital disc reader, in particular a DVD reader.

[0043] Generally speaking, the invention synchronizes two clock signals using fast and cheap digital means.

[0044] As can be seen in FIG. 1, the device 1 for generating a synchronization variable WCS receives as input a phase variation signal ΔΨ which can be generated as explained later with reference to FIG. 4, and a period signal P_(wp1) of the clock signal WP₁, at a frequency of 20 MHz, for example. The synchronization signal WCS at the output of the device 1 is fed to a module 2 which also receives as input a clock signal WP₂ and is capable of supplying at its output a synchronized clock signal WP_(2s) whose frequency is equal to that of the clock signal WP₂ and which is synchronized to the signal WP₁.

[0045] A module, not shown, can be provided for calculating an approximation P_(wp) of the period of the second clock signal WP₂. The following condition applies: P_(wp)=B/ΔΨ, where B is a constant that depends on the number of bits used for the calculation, the frequency of the signal WP₁, a sampling frequency, for example 400 MHz, and a writing frequency, for example 130 kHz.

[0046] To simplify the calculations, P_(wp) can be pre-calculated for intervals of ΔΨ.

[0047] The device 1 includes a module 3 for calculating the phase Ψ from the phase variation signal ΔΨ and the period P_(wp1). The module 3 adds the phase variations of the preceding times and the phase variation of the current time modulo the period P_(wp1). In other words, the module 3 performs the following operations:

[0048] Ψ_(n+1) takes the value Ψ_(n)+ΔΨ_(n+1);

[0049] if Ψ_(n+1)≧Ψ_(max), then Ψ_(n+1) takes the value Ψ_(n+1)−Ψ_(max).

[0050] Ψ_(max) is the maximum value of the phase, i.e. its value at time P_(wp1).

[0051] The phase Ψ can be determined from a phase variation signal coming from another clock, not shown, at a different frequency but with a particular phase. To simplify the calculations, the phase Ψ can be calculated every p periods of the first clock signal WP₁, where p>1.

[0052] The device 1 includes a module 4 which receives the phase signal Ψ from the module 3 and the approximate period P_(wp1). The module 4 determines a first approximation WS₀ of the synchronization variable WCS, with: WS₀=Ψ×P_(wp)/A, where A is a constant. The value of A is predetermined as a function of the clock signal to be synchronized and will be taken as equal to a power of 2. For example, the value A=2¹⁵ can be taken.

[0053] The device 1 includes a module 5 for determining other approximations close to the first approximation WS₀. The module 5 is connected to the output of the module 4. The module 5 performs an addition operation to generate a second approximation WS_(p1), and a subtraction operation to generate a third approximation WS_(m1), with WS_(p1)=WS₀+1 and WS_(m1)=WS₀−1. Three approximations are obtained at the output of the module 5, the first one lying between the second and third ones so that their respective precisions can be compared.

[0054] The device 1 includes a module 6 for calculating errors which receives the first approximation WS₀ from the module 4 and the second and third approximations WS_(p1) and WS_(m1) from the module 5. The module 6 also receives the phase variation signal ΔΨ. The module 6 calculates three error values err_(—)0, err_p1 and err_m1 respectively corresponding to the first, second and third approximations WS₀, WS_(p1), and WS_(m1), where:

err _(—)0=(B*Ψ)−(C*ΔΨ*WS ₀)

err _(—) p1=(B*Ψ)−(C*ΔΨ*(WS ₀+1))

err _(—) m1=(B*Ψ)−(C*ΔΨ*(WS ₀−1)), and where B and C are constants.

[0055] It will be noted that calculating error values uses addition, subtraction and multiplication operations, but no division operations, which are slow and costly in terms of calculation resources. The constants B and C are predetermined as a function of the signals to be synchronized. The following values can be taken, for example: B=20 and C=13.

[0056] The device 1 includes a selector module 7 which receives the three error values err_0, err_pl and err_ml from the error module 6, the first approximation WS₀ from the module 4, and the second and third approximations WS_(p1) and WS_(m1) from the module 5. The module 7 determines which of the aforementioned errors is the smallest one and sends to its output the approximation subject to the smallest error. Said approximation is sent to the output of the device 1 as the synchronization variable WCS. In other words:

[0057] if err−0=min (err_(—)0, err_p1, err_m1), then WCS=WS₀;

[0058] if err−p1=min (err_(—)0, err_p₁, err_m1), then WCS=WS_(p1);

[0059] if not WCS=WS_(m1).

[0060] The device 1 is advantageously integrated and constitutes a portion of an integrated circuit.

[0061]FIG. 2 shows steps performed in chronological order to execute the method.

[0062] Step 10 calculates an approximation P_(wp) of the period of the second clock signal.

[0063] Step 11 calculates the phase v as a function of the phase difference ΔΨ and the period P_(wp1).

[0064] Step 12 calculates the first approximation WS₀ as a function of the phase Ψ and the approximate period P_(wp).

[0065] Step 13 calculates the second and third approximations WS_(p1) and WS_(m1) from the first approximation WS₀.

[0066] Step 14 calculates the errors corresponding to each of the aforementioned approximations.

[0067] Step 15 takes as the synchronization variable the approximation with the smallest error.

[0068]FIG. 3 shows various signals described above. The clock signal W_(p1) has a frequency of 20 MHz. The vertical lines 8 correspond to the upward transitions of the clock signal W_(p1). Between two downward transitions, the signal W_(p2) defines a writing period that has to be determined. The signal W_(p2) can be entirely determined knowing the upward transitions of the signal W_(p1) and the synchronization variable WCS, which can be defined as the time difference between a downward transition of the signal W_(p2) and the next upward transition of the signal W_(p1).

[0069]FIG. 4 shows parts of a DVD reader 20, which parts are useful for describing means for calculating a phase difference ΔΨ.

[0070] An optical sensor 21 is made up of four photodetectors 22, 23, 24, 25. In practice, a laser diode emits a laser beam that is directed via a splitter plate and a conventional optical system onto the face of the disc on which is etched the track containing the data. The beam reflected by the disc then passes through the splitter plate in the opposite direction and is picked up by the four photodetectors 22, 23, 24, 25, all four of which are in a plane parallel to the plane of the disc. Each photodetector delivers a signal which, for simplicity, is identified by the same number as the photodetector. The individual signals 28, 29, 30, 31 are then processed in a first processor stage 26 having at its input a preliminary processor module 27 including amplifiers. The structure of the module 27 is known in the art.

[0071] The signals 28, 29, 30, 31 delivered by the module 27 are then sampled in four analog-digital converters 32 to 35. The sampling clock signal is generated using a quartz crystal, for example (not shown here for simplicity).

[0072] The two individual signals obtained from the two photodetectors that are symmetrical with respect to the top center point of the sensor are then summed in two adders. One of the secondary signals is equal to the sum of the individual signals 28 and 30 and the other secondary signal is equal to the sum of the individual signals 29 and 31.

[0073] The summed signals are then filtered in respective identical low-pass filters 36 and 37. The processor stage therefore supplies two sampled incident signals whose phase difference is representative of the positioning error of the optical beam relative to the track on the disc. The two sampled signals are processed by the phase difference calculation device according to the invention incorporated in the stage 38, the latter device delivering a positioning error that is used in the conventional way in a control loop to modify the incident optical signal and return the optical beam to the track on the disc and lock it there. The positioning error can be taken as a phase difference or subjected to intermediate processing before it is used by the device 1 shown in FIG. 1.

[0074] The skilled person knows that the optical sensor formed of the photodetectors delivers, in addition to the four individual signals, a global signal, or wanted signal, equal to the sum of the four individual signals, from which wanted signal the binary data read from the track is extracted. The processor means for extracting the coded data that are conventionally part of a digital disc reader are not shown, for simplicity. 

1. A method of generating a synchronization variable WCS intended for a second clock signal WP₂ from a first clock signal WP₁ and a phase variation signal ΔΨ, in which method: a first approximation WS₀ of the synchronization variable is determined, other approximations close to the first approximation are determined, the error is calculated for each of the approximations, and the best approximation is taken as the synchronization variable.
 2. A method according to claim 1, wherein, prior to determining the first approximation WS₀ of the synchronization variable, an approximation P_(wp) of the period of the second clock signal WP₂ is determined.
 3. A method according to claim 1 or claim 2, wherein a second approximation WS_(p1) and a third approximation WS_(m1) of the synchronization variable are determined, where WS_(p1)=WS₀+1 and WS_(m1)=WS₀−1.
 4. A method according to any preceding claim, wherein the phase Ψ is calculated at a given time by adding phase variation signals ΔΨ of preceding times and of the given time modulo the period of the clock signal WP₁.
 5. A method according to claim 4, wherein the first approximation WS₀ is obtained from the following equation, in which A is a constant: WS₀=Ψ*P_(wp)/A.
 6. A method according to claim 4 or claim 5, wherein error values are determined corresponding to the approximations effected: err _(—)0=(B*Ψ)−(C*ΔΨ*WS ₀) err _(—) p1=(B*Ψ)−(C*ΔΨ* (WS ₀+1) err _(—) m1=(B*Ψ)−(C*ΔΨ*(WS ₀−1)), where B and C are constants.
 7. A method according to claim 6, wherein the synchronization variable WCS is equal to the approximation generating the smallest error.
 8. A method according to claim 5, in which A is equal to 2^(n) where n is a predetermined constant.
 9. A device for generating a synchronization variable WCS intended for a second clock signal WP₂ from a first clock signal WP₁ and a phase variation signal ΔΨ, characterized in that it includes: means for determining a first approximation WS₀ of the synchronization variable, means for determining other approximations close to the first approximation, means for calculating the error for each of the approximations, and means for taking the best approximation as the synchronization variable.
 10. An integrated circuit including a device according to claim
 9. 11. A digital disc reader including an optical head including means for emitting an incident light beam and a plurality of photodetectors (22 to 25) for detecting the reflection of the light beam at the disc, summing means for summing signals from the photodetectors two by two and delivering two sampled incident signals, and calculator means (38) for calculating the phase difference of the two sampled incident signals, characterized in that the calculator means (38) include a device as defined in claim
 9. 